The technical definition is a person who visits the disney theme parks. But it has a more subtle definition that is more accurate.

Generally Under-educated Egotistical Stupid Tourists

The way the guest comes up to the cast members who work for Disney and demand things that are not available or they perform acts that are unsafe to themselves of others and then complain when the cast member asks them nicely to stop for their safety. (for example: slacking their seat belts on tower of terror even when the cast member has told them not to.... Also how the Annual Pass Holders think they know the attraction better than the people that actually work there every day.